Multi-tenant datacenters typically have numerous network devices, such as routers and switches, which are interconnected to provide service and connectivity to the various tenants in the datacenter. In particular, many of these network devices provide network connectivity to tenant infrastructure, while others are connected to servers and other devices in the tenant space to provide application and network services. Some application and network services can include, for example, DHCP, NTP, firewall and security services, policy services (e.g., Policy Gateway servers), and so forth. As such, these network devices provide important services in the datacenter, and any interruption in service can be extremely disruptive and inconvenient.
However, many times, the various devices in a datacenter must undergo upgrades, which can result in downtime for the upgraded devices and thus the services they provision. Not surprisingly, image upgrades of network devices in the datacenter can be very difficult to implement, as each image upgrade often results in service disruptions. This challenge can become even more profound as the number of network devices in the datacenter—and consequently the potential service disruption—increases. Unfortunately, many network devices in a datacenter do not fully support ISSU (in-service software upgrade), which would reduce the service disruption period from upgrading images, and must therefore undergo some period of downtime.